This invention relates to a reverse conducting thyristor having a thyristor section, a diode section and a semiconductor separator section.
A reverse conducting thyristor comprises a thyristor section, a diode section and a seimconductor separator section which are formed integrally. The seimconductor separator section is interposed between the thyristor and diode sections and prevents electrical interference therebetween. Such a reverse conducting thyristor is employed chiefly as a chopper device or an inverter device, and should therefore have a great turn-off ability. Since a large current flows through it, the thyristor section should have a large effective area. In addition, the effective area of the diode section should be made sufficiently large. To improve the turn-off ability of the thyristor, it is necessary to shorten the turn-off time and to prevent the thyristor section from being refired by residue charge in the diode region.
To facilitate the understanding of a reverse conducting thyristor, a chopper circuit including reverse conducting thyristors will be explained with reference to FIGS. 1, 2A and 2B.
In the chopper circuit shown in FIG. 1, a main reverse conducting thyristor I is connected through a load M between the positive and negative poles of a DC power source. Between the terminals of the thyristor I there is connected an auxiliary thyristor II through an inductor L and a capacitor C. First, a gate signal is supplied to the gate of the thyristor I, thus making current I.sub.DC flow through the load M. Then, a gate signal is supplied to the gate of the thyristor II. As a result, the thyristor I is turned off while the thyristor II is rendered conductive.
While current I.sub.DC is flowing through the load M, the capacitor C is charged. If the auxiliary thyristor II is turned on under this condition, both current I.sub.DC and the current discharged from the capacitor C flow through the thyristor section of the main thyristor I for a period T.sub.0 as shown in FIG. 2A. In period t.sub.p1 the polarity of the capacitor C is reversed. Then, the current discharged from the capacitor C flows through the diode section of the main thyristor I, and the thyristor I is turned off. A pulse current as shown in FIG. 2B flows through the thyristor and diode sections of the auxiliary thyristor II during a time period t.sub.p2, respectively. Since the current flowing through the diode section of the main thyristor I is smaller by current I.sub.DC than the current flowing through the diode section of the thyristor II, the pulse width of the pulse current of the diode section of the thyristor II becomes extremely small.
If either thyristor fails to be turned off as in case it is re-fired, the pulse current continues to flow through the thyristor sections of both thyristors I and II. In this case, the pulse current change rate di/dt is very high, and the thyristor section of each thyristor should withstand such a high current change rate di/dt. To this end, the thyristor section of each thyristor should be fired very rapidly; its firing area should expand very quickly.
Through the diode section of either thyristor there flows a pulse current having a small pulse width. This pulse current is therefore doomed to attenuate very quickly. The quick attenuation of the current adversely builds up a residue charge in the diode section. The residue charge thus built up may often cause the thyristor section to be re-fired. To prevent such a re-firing of the thyristor section, the separator section is provided between the thyristor and diode sections.
On the other hand, through the thyristor section of thyristor II flows such a large current as shown in FIG. 2A. It is therefore required that the effective area of the thyristor section should be made larger in order to reduce the voltage drop in forward direction as much as possible. Apparently, the effective area of the thyristor should be increased if a great current is to flow through the thyristor section.
A conventional reverse conducting thyristor has such a construction as shown in FIGS. 3A to 3C. FIG. 3A is an upper plan view of the conventional thyristor, and FIG. 3B is a cross-sectional view of the thyristor taken along line 3B--3B in FIG. 3A. As illustrated in FIG. 3B, there is mounted on the central part of an anode electrode 1 a cylindrical diode section 2 which is constituted by an N.sup.+ layer, an N layer and a P layer. The diode section 2 is surrounded by a hollow cylindrical separator section 3 which is constituted by a P.sub.1 layer, an N layer and a P.sub.2 layer. On the periphery of the separator section 3 a ring-shaped thyristor section 4 is provided. The thyristor section 4 is constituted by a ring-shaped P.sub.1 layer, a ring-shaped N layer, a ring-shaped P.sub.2 layer, a main emitter region 5 and an auxiliary emitter region 6. The P.sub.1 layer is formed on the anode electrode 1, the N.sub.1 layer on the P.sub.1 layer, and the P.sub.2 layer on the N.sub.1 layer. The main emitter region 5 is so formed in the P.sub.2 layer as to have its surface at the same level with that of the P.sub.2 layer. The auxiliary emitter region 6 is formed similarly in the P.sub.2 layer and faces at least a part of an outer peripheral of the main emitter region. On the main emitter region 5, the separator section 3 and the diode section 2 a cathode electrode K is formed. Further, an auxiliary gate electrode 7 is provided around the cathode electrode K and touches the auxiliary emitter region 6, and a main gate electrode G is connected to that surface portion of the P.sub.2 layer which is nearer to the auxiliary emitter region 6 than to the main emitter region 5.
In the reverse conducting thyristor of the abovementioned construction, the auxiliary gate electrode 7 surrounds the entire periphery of the main emitter region 5, and the separator section 3 is a hollow cylinder. It is therefore difficult to increase the effective areas of the main emitter region 5 and the diode section 2, through which the thyristor current and the diode current are to flow, respectively. That is, if the outer radius of the auxiliary gate electrode 7, the outer radius of the cathode electrode K, the outer radius of the separator section 3 and the inner radius of the separator section 3 are denoted by R.sub.1, R.sub.2, R.sub.3 and R.sub.4, respectively as shown in FIG. 3C, the ineffective area where neither thyristor current nor diode current flows is very large as hatched in FIG. 3C. Since the effective area of, for example, the thyristor section 4 is so small that the voltage drop in the forward direction will become too high.
Accordingly, the object of this invention is to provide a reverse conducting thyristor whose turn-off time is shortened and whose thyristor and diode sections have a relatively large effective area where current may flow.